The objectives of the proposed research are to provide a description of the neural and photochemical events subserving visual adaptation in the vertebrate retina, to examine the role of the various retinal elements in the adaptation process, and to elucidate the synaptic mechanisms by which information is transferred from the receptors to more proximal neurons. The visual pigments of their photosensitive reactions will be studied in situ using the methods of transmission spectrophotometry and fundus reflectometry, and kinetic data obtained from these studies will be compared with the changes in electrophysiological thresholds to various adapting conditions. Using ionselective electrodes, the light-induced changes in extracellular potassium will be measured in order to determine whether this ionic species relates to the excitability changes associated with adaptation and generation of specific components of the electroretinogram. Intracellular recordings and analysis of drug-induced changes in receptive field organization will be used in the identification of chemical agents mediating signal transmission in the distal retina. In other experiments, we plan to utilize autoradiographic and electron microscopic techniques to examine the structural and functional properties of synaptic terminals, i.e., binding sites, membrane specializations and the release of synaptic vesicles.